Light Gap

The light gap is the daylight that rings an inside-mounted shade along the sides and bottom. It exists because the width deduction leaves clearance between the fabric and the frame, and because most shades don’t seal to the sill. This is why “blackout” describes the fabric, not the installed result: mount a blackout shade inside the frame and light still outlines it. The fixes are structural, not fabric-based — an outside mount with generous overlap, side channels, or a dual-shade approach. Any honest bedroom or nursery recommendation names the gap up front rather than promising a dark room from an inside mount alone.